1. Field of the Invention:
The present invention relates to a novel process for the preparation of amorphous (co)polyamides from at least one aromatic dicarboxylic acid containing from 8 to 18 carbon atoms and from at least one linear or branched chain aliphatic diamine containing from 6 to 12 carbon atoms.
2. Description of the Prior Art:
Wholly aliphatic linear polyamides of the nylon type are known to this art, of high molecular weight, and having many physical and chemical properties which make them suitable for the production of a wide variety of useful shaped articles such as fibers, films and other molded objects. Polyhexamethylene adipamide or nylon 66 is an example of a polyamide which has been extensively developed.
However, these polymers exhibit a certain number of drawbacks, the magnitude of which is a function of their intended use, such as, for example, a mold shrinkage which can be significant, a water regain which may be considerable and, as a corollary, a dimensional stability towards moisture which may be insufficient, and mechanical properties which are adversely affected by environmental moisture which decrease, furthermore, at temperatures above 100.degree. C.
Very many patents describe polyamides containing different groups such as, especially, polyamides containing mixed aliphatic and aromatic recurring units. The introduction of aromatic rings provided an increase in the melting or softening point and in the glass transition temperature, with a consequently improved retention of mechanical properties, especially at high temperatures. Typical polyamides of this type are, for example, the amorphous copolymers produced from isophthalic acid (60% to 90 mol % in the mixture of diacids), terephthalic acid and hexamethylenediamine (cf. U.S. Pat. No. 3,382,216).
To prepare such amorphous (co)polymers, the prior art proposes, for example, to adopt the solution polycondensation technique, according to which the halide(s) of the dicarboxylic acid(s), such as, for example, the chloride(s) of diacid(s) is(are) reacted with the diamine(s), the operation being carried out in a solvent devoid of any traces of water. For a greater efficiency of the polycondensation, it is also necessary to dissolve in the reaction medium an organic base capable of reacting with the hydracid released during the acylation reaction. Once the polycondensation is complete, the polymer is isolated, generally by precipitation in a nonsolvent. This type of process is not of great industrial interest, especially because of the fairly high cost of the starting dihalides of aromatic carboxylic acids and of the complexity of the apparatus, which must permit the handling of large volumes of liquids in relation to the amount of polymer produced.
Another known method of preparation of (co)polyamides based on aromatic dicarboxylic acid(s) and on aliphatic diamine(s) entails the classical nylon 66 type of bulk polycondensation process in a melt. Stoichiometric amounts of diamine(s) and of dicarboxylic acid(s) are introduced, water being added if appropriate, into a stainless steel autoclave. It is sometimes advantageous to prepare the salt of the diamine(s) and dicarboxylic acid(s) beforehand. The reactants are heated to a temperature ranging from about 200.degree. to 220.degree. C., under stirring. The water present is removed and the temperature is increased to approximately 250.degree. to 280.degree. C. At this temperature, stirring of the reaction mixture is continued for some time at atmospheric pressure and ultimately at a reduced pressure, in a stream of nitrogen, until the time when the polymer has attained the desired molecular weight and viscosity. In order to obtain, upon completion of this stage, (co)polyamides which, after molding, provide shaped articles having good mechanical properties, it is generally necessary to continue the polycondensation until the time when the (co)polymer has attained viscosity characteristics, expressed in the form of the viscosity index (VI), measured in meta-cresol at 25.degree. C. on a solution containing 0.5 g of dried polymer in 100 cm.sup.3 of solvent, which are at least equal to 100 ml/g. However, in this viscosity range, the following problem is encountered: the (co)polyamide flows with difficulty in the molten state and, as a result, is difficult to drain properly from the autoclave in which it is prepared. More precisely, the draining pressure is high, the draining time is long and the proportion o degree of draining is low.